This invention relates generally to condition-responsive switches and, more particularly, to a refrigerator thermostat with a reduced spring load in the off position.
Switches that are responsive to temperature changes, commonly known as thermostats or cold controls, are used in refrigeration appliances, such as refrigerators and freezers, to control the temperatures therein. These thermostats regulate the switching cycle of the refrigeration compressor in response to the temperature of the air contained at some location within the appliance. When the temperature exceeds a certain "turn-on" point, the switch contacts are closed and the compressor is switched on to cool the appliance. When the temperature drops below a certain "turn-off" point, the switch contacts are opened and the compressor is switched off.
One type of thermostat utilizes a bellows communicating with a capillary tube in thermal contact with the location to be cooled. Expansion and contraction of a gas within the capillary tube and bellows causes the length of the bellows to expand and contract. The motion of the bellows moves a pivoting actuator arm to open and close switch contacts to turn the compressor on and off. The actuator arm is biased with a spring to counteract the bellows and calibrate the thermostat so that the compressor will be turned on and off at specified temperatures. The bias of the spring is typically preset at the factory where the switch is manufactured, but is adjustable by a user so that a refrigeration compartment may be kept warmer or cooler as desired.
User adjustment of the set point of such a thermostat is accomplished via rotation of a shaft that extends through a thermostat housing. As the shaft rotates, a cam connected to the shaft also rotates, and a cam follower contacts a surface of the cam to increase or decrease the load on the actuator arm bias spring that connects the cam follower and the actuator arm.
Sometimes, it is desirable to open the switch contacts and prevent the compressor from cycling on and off, such as during storage, cleaning, and defrosting of refrigeration appliances. This is accomplished by shaping the cam surface so that the spring attached to the cam follower is increasingly loaded as the cam follower follows the cam surface to an OFF position. In such a position, the bellows may not generate sufficient force to overcome the spring and close the switch contacts, and by either positioning the actuator arm in a "manual off" position to ensure the separation of the switch contacts, or by opening a secondary pair of contacts known as a "line switch," it can be ensured that the compressor will not be switched on. In a manual off thermostat, the increased cam profile causes the cam follower to push a manual off leg that is connected to the actuator arm. This places the actuator arm in a pivoted position separating the switch contacts. In a line switch thermostat, a cam flag attached to the cam contacts and moves a push rod which separates a secondary pair of contacts and breaks the circuit through the thermostat.
In both manual off and line switch off thermostats, the increased load on the actuator arm bias spring is undesirable. The increased load on the bias spring leads to spring load loss that negatively affects the calibration of the switch when the thermostat is returned from an OFF position to a desired WARM or COLD setting. Thus, the thermostat is rendered inaccurate, the refrigeration compartment runs warmer or colder than desired, and user adjustability of the ON and OFF compressor temperatures is compromised.
Accordingly, it would be desirable to provide a thermostat which reduces or eliminates the bias spring load in an manual OFF or line switch OFF position.